The present inventive subject matter relates to lighting apparatus and methods and, more particularly, to solid-state lighting apparatus.
Solid-state lighting arrays are used for a number of lighting applications. A solid-state light-emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs). These may include inorganic LEDs, which may include semiconductor layers forming p-n junctions, and/or organic LEDs (OLEDs), which may include organic light emission layers.
Solid-state lighting devices are commonly used in lighting fixtures, including task lighting, recessed light fixtures, ceiling mounted troffers and the like. Solid-state lighting panels are also commonly used as backlights for small liquid crystal display (LCD) screens, such as LCD display screens used in portable electronic devices, and for larger displays, such as LCD television displays.
Solid-state lighting devices may be attractive for retrofit/replacement applications, where devices such as LEDs may offer improved energy efficiency, reduced heat generation, extended life and desired performance characteristics, such as certain color and/or color rendering capabilities. For example, LED bulbs are commonly used to replace incandescent bulbs in down lights and other applications to reduce energy consumption and increase time between replacements. LED-based replacements for fluorescent lamps have also been developed.
An LED driver may use a pulse width modulated current source to control current through a string of LEDs. Current through the string may be controlled by a current loop operating at a relatively high PWM frequency, with a duty cycle of a current switch of the current source being varied to regulate the current through the LED string. Output intensity of the LED string may be further controlled by superimposing a second frequency modulation in response to a light intensity control input, such as a dimming input. In particular, this modulation may periodically turn off the current through the LED string responsive to a dimming input. This second modulation may occur at a much lower frequency than the current loop PWM switching frequency.